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Why increase the number of cylinders in an engine instead of increasing their volume?
I've been reading about WW2 planes lately. Some of them have 12 or even more pistons in their engines.
But if your goal is to increase power, why would you add more pistons instead of simply increasing the size of the cylinders?
Example: The P-51 Mustang had a V12 engine with a total displacement of 27 liters. So that's 2.25 L per cylinder. Instead, why not have a V4 engine with the same total displacement, which would have been 6.75 L per cylinder?
Correct me if I'm wrong, but...
Such "collectivizing" of the cylinders would be more efficient for a number of reasons. The friction of piston rings scraping the cylinder would be less, meaning less oil and more power. The crankshaft and related things could be shorter. I believe it would be lighter for another reason too: the surface area goes up less than the volume (squared versus cubed). Probably the engine as a whole would be simpler.
I believe this concept can be applied to radial engines as well as inline engines. I made an assumption that 4-stroke engine needs a minimum of 4 cylinders. Today that is not true but during WW2 with the technology of the time, I'm not sure. There were radial engines with 3 cylinders. I'm unaware of any 4-stroke engines with 2 or less cylinders in that era.
Anyway, why not just make the cylinders bigger?

This isn't really specific to aviation. The same thing is done in car engines.

@reirab I would argue it's more specific to aviation just because it would save weight. Weight is more important for aircraft than, say, car or train engines. Nevertheless, if it applies to aviation, I hope it's on topic here regardless if the principle applies to other fields.

Oh, yeah, I wasn't intending to imply it's off-topic, just commenting that other applications seem to make the same choices, so it's likely that the primary reason(s) aren't specific to aviation.

One reason is that you can build a larger engine just by adding more cylinders, using the same pistons, rods, &c.  So a 4-cylinder O-360 becomes a 6-cylinder O-540.  Other (possible - I'm not an aviation engineer) reasons might include things like rotating inertia & volumetric efficiency.  Consider the difference between the V-twin of a Harley-Davidson, and the high-revving 4 & 6 cylinder engines used by most of the competiton.

@jamesqf your motorbike analogy is a good one. Bigger cylinders means a lot of torque at low rpm. More cylinders generally gives a nicer torque distribution. Also there's the vibration issue, a few big cylinders won't counter each other a nicely as several smaller cylinders.

an important reason is that increasing cylinder volume does not increase horse powers in direct proportion. Increasing the number of cylinders does, more or less. IOW 12 2l cylinders produce more power than 6 4l cylinders.

Ever tried a V8 car? Big difference from a normal 4 inline...

@Fabrizio Mazzoni: Driven a good many, back in the day when V8s were common.  (And even a Buick straight 8.) Also have had a good many inline 4s, in cars (mostly sports cars), trucks, and motorcycles, several opposed 4s in Subarus and airplanes, and my main vehicle nowadays is an inline 3 cylinder.  Can't really tell much difference.

@jwenting Can you explain why? If the total displacement and total fuel injection are the same, I see no reason why horsepower would not be the same or even a little bigger. 4 cylinders should have less friction and less inertia than 8 or 12.

I was thinking: reliability.  If one thing goes wrong with a cylinder -- ignition, fuel injector, valve -- that whole cylinder is out of commission.  In a 4-cylinder, that means at least a 25% loss; in a 10-cylinder, just 10%.

Shipbuilders did go in the direction of bigger cylinders rather than more cylinders, and the result gives an indication of why they're not used for airplanes: a large marine diesel might operate at 80 RPM or slower.

It depends on what you mean by "minimum" but a 4 stroke engine needs a minimum of 6 cylinders, not 4. With 1 cylinder (the minimum minimum) the engine is out of balance, with 4 it is out of balance with 8 it is out of balance. 4 stroke engines are only balanced properly with 6 or 12 cylinders

@slebetman: I've been driving a car (Honda Insight) for the last dozen years that has a 3 cylinder, 4 stroke engine.  Plenty of bikes (notably Harleys) have 2 cylinders, many of today's cars and my Cherokee's O-360 have 4.  Audi even had (and perhaps still does) a 5 cylinder engine.  Others have V-10 and V-12 engines.  So the minimum number of cylinders needed seems to be I.

@jamesqf Yes that's for today. I will edit the OP clarifying that for the WW2 era, 3 or 4 seems to be a minimum. Who knows if someone will find a counter-example, so I could still be wrong.

@DrZ214: WW2 era motorcycles tend to be 1 cylinder

@DrZ214 with larger cylinders, the mixture becomes harder to control, especially at higher rpm (thus less time for the mixture to stabilise in the cylinder. Thus you end up with uneven combustion, which is less efficient. Big cylinders are ok for low rpm engines, providing high power per stroke but few strokes, not so much for high rpm engines (which you want in aircraft to have a constant amount of power, rather than a spikey output).;

@DrZ214: I don't know about actual practice in WWII, especially in airplanes.  I was responding to the claim that 4-stroke engines have to have a minimum of 6 cylinders.  It's true that 6 or 12 may balance better, but that's certainly not a requirement.

Not really an answer, but once you have a well-designed radial engine, you can increase its power simply by adding another row, without having to redesign the cylinders or even change the engine cowling much. The example that springs to mind is http://www.pw.utc.com/R4360_Wasp_Major_Engine -- developed during WWII, with four rows of seven cylinders each, for a total of 28 cylinders!

Constraints
Different applications have different constraints:

Aviation: very light weight, highly reliable
Marine: very high endurance
Automotive: moderately light weight, responsive
Motorcycle: very light weight, very compact, very responsive

Different technology ages yield different solutions due to additional constraints, always limited by the then contemporary technology:

Pioneer era: make it work
World War I/II era: as fast as possible
Post-war era: further, faster, better
Fuel crisis era: as efficient as possible

Aircraft Engines
The question is about the optimization of number of cylinders versus displacement volume per cylinder for engines used for aviation. This narrows the scope to “internal combustion reciprocating piston engines” (plus the Wankel engine as a very special case).
Obviously, rockets, pulse jets, turbine-powered,  and electric engines have no cylinders, and steam engines were never  (successfully) used in aircraft.
Number of cylinders and the cylinder displacement are two out of countless parameters that go into the design of any engine. Both may be used to increase the power output.
Power Increase
The power output of an engine may be increased either through the number of cylinder or through increasing the cylinder displacement (or both).
Each change of parameters causes the gain or loss of certain wanted characteristics. These are listed further below under (N), (n), (D), and (d).

Increasing the number of cylinders means gaining (N) and losing (n)
increasing the cylinder displacement means gaining (D) and losing (d)

Adding cylinders is easier than increasing the size of the cylinder. The cylinder geometry does not change. Identical engine parts can be used multiple times in the same engine design (cylinder banks, cylinder heads, or complete engine blocks).
Tradeoff shift
Starting from one engine configuration, the same power output may be achieved by

gaining (N) and (d), and losing (n) and (D)
or
gaining (n) and (D), and losing (N) and (d).

Reasons to increase the number of cylinders (N)

Torque directly scales with the number of cylinders
Increasing the surface-to-volume ratio is advantageous for air-cooled engines
Increase the power: Adding cylinders is easier than increasing the size of the cylinder. The cylinder geometry does not change. Identical engine parts can be used multiple times in the same engine design (cylinder banks, cylinder heads, or complete engine blocks)
Improve balancing of forces and momenta
Reduce the time between power strokes
Decrease the impact of a failing cylinder
Improve the flatness of the torque distribution over revolution speed.
Enable more flexible and more distributed form factor

 Pratt & Whitney R-4360 Wasp Major, 28-cylinder, 28 l, 3500 hp, 2700 rpm, built 1944-1955.
Reasons to decrease the number of cylinders (n)

Simplicity: less moving parts improve robustness,  decrease the need for service, thereby increase the availability.
Enable a more compact form factor

Mercedes 1 cylinder, 1.5 kW, 720 rpm, 84 kg, built 1888.
Reasons to increase the cylinder displacement (D)

Increase power through torque

BMW IIIa, 6-cylinder, 19.1 l, 200 hp, 1400 rpm, built 1917.
Reasons to decrease the cylinder displacement (d)

Smaller displacement means smaller pistons, shorter rods, or both. Either way,  smaller displacement allows for higher revolution speed, and higher acceleration.
Smaller combustion chamber will decrease the time required for the flame expansion (gasoline only, not diesel). This allows for higher revolution speed.
The valves are limiting the gas stream into and out of the cylinder. The valves are subject to the surface-volume ratio. Smaller cylinders are easier to fill and empty through the valves, allowing for higher revolution speed.
At a given compression rate, smaller cylinders have to withstand less total force, allowing for a lighter engine structure (less weight).

JPX PUL 212, 1 cylinder, 212 cm³, 11 kW, 6000 rpm.

Notes

Radial engines belong to the WW I/II era. Most of them were air-cooled. For air-cooled engines, the surface-to-volume ratio matters. Therefore increasing the number of cylinders instead of the displacement per cylinder is obvious.

Aircraft during WW I/II had to be as fast and powerful as possible to attack and defend. There was no good reason to go for less than 6 cylinders.

Four-stroke engines perfectly work with 1, 2 and 3 cylinders. They are used powered paragliders respectively ultralight aircraft.

Certain cylinder numbers are more preferable to due symmetry reasons

6, 8, 4 for in-line engines
odd numbers (per row) for radial engines

Building radial engines with an even number of cylinders is well possible, albeit an even number in one row us not preferable. Multi-row radial engines with even cylinder number have been flown in many aircraft.

Automotive engine developers prefer 0.5 l per cylinder as ideal trade-off.

A high cylinder count would be necessary to build high power piston engines, but this segment is now occupied by jet engines.

Radial engines with less than 5 cylinders exist. Here is a radial 3-cylinder, built 1930 in the USA:

It was probably easier to add cylinders (fewer design decisions to review, fewer drawings to alter, fewer tooling changes) than to increase displacement.  Increasing displacement means more or less designing a new engine vs. making modifications to an existing design.

@DanPichelman Just as with CPU power on modern mainboards. It's easier to just add more "phases" (up to 40) that can be shared with 4-phase budget boards instead of designing more powerful single "phase" to lessen problems with synchronizing them. The availability of high-power mosfets and individual cooling also plays a role. The similarities to engine cylinders is striking ; )

cool photo !!!!

"Either way, larger displacement will limit the maximum revolution speed." Indeed, look at the opposite: a Yamaha R1 engine has tiny cylinders with a redline of 14,500 rpm!

An easy mistake to make, but that's not a Dutch engine: the Szekely Aircraft & Engine Co was headquartered in Holland, Michigan. Also it was nicknamed the Flying Dutchman, just to confuse us :-)

That is hilarious. Thanks!

Your reasoning is correct if engine mass is not important. Ships use huge engines, because increasing the number of cylinders beyond 8 will have diminishing returns in terms of smoothing out the torque ripples, and bigger cylinders help to increase efficiency. But aircraft need to keep the mass of the engine down.

Wartsila-Sulzer RTA96-C turbocharged two-stroke diesel engine during assembly (picture source). Its size makes this engine supremely efficient: Its 14-cylinder version produces 108,920 hp at 102 rpm and has a thermal efficiency of more than 50%. Specific fuel consumption is only 0.260 lbs/hp/hour. But it weighs 2600 tons!
Engine power is the product of torque and speed. To maximize engine power, the speed must be kept as high as possible. Increasing cylinder size will limit the speed at which the engine can be run due to the speed of the combustion process inside the combustion space. If the cylinder diameter grows too big, the flame front originating from the spark plug will not have traveled far enough to have burnt most of the fuel by the time the piston moves down again. Only adding more cylinders will increase power while keeping the speed of the engine constant.
Here is a comparison of WW I aircraft engines from the excellent enginehistory.org site. Note how the figures for bore and speed correlate inversely (the Austro-Daimler 120 was a pre-war design and saw later speed increases):

Graphical comparison, the Austro-Daimler is shown with the specs of a later version.
Quote from the linked PDF (enginehistory.org):

The large bore diameter, however, pushed the upper limit of an
  aero-engine cylinder. Adequate cooling and fuel efficiency require a
  complete as possible combustion of the fuel-air mixture and this
  complete combustion requires that the flame fronts moving across the
  combustion chamber from their respective points of ignition be
  given time to meet. The speed of a four-stroke aero-engine with a
  large cylinder bore is thus actually limited by the rate of combustion
  of the fuel-air mixture which for a given cylinder and mixture is a
  constant and thus efforts to increase the output horsepower by
  increasing the speed of an engine with a large bore cylinder may
  result in incomplete combustion, over-heating and detonation.

Other limits to engine speed like loads on the connecting rods or adequate cylinder filling and flushing can be dealt with by using materials of higher strength and more valves per cylinder, respectively, but when the type of fuel is given, the hard limit for engine speed is the cylinder's bore. So the only way to increase power without hurting the power/weight ratio is to add more cylinders.

If the cylinder diameter grows too big, the flame front originating from the spark plug will not have traveled far enough to have burnt most of the fuel by the time the piston moves down again. If I'm interpreting this correctly, then diesel engines will not suffer this problem. No matter the volume of the cylinder, the auto-ignition conditions will be met by the entire volume once the piston moves down far enough. So there is no "speed of flame front" in that case. Is this valid?

P.S., this is just an off-topic aside, by why would a huge marine engine use 2-stroke instead of 4-stroke? 2-stroke combines the exhaust phase with the fuel intake phase, and therefore some of the income fuel escapes out the exhaust without being burnt. I thought 2-strokes were only best for very small applications like small watercraft, not huge marine vessels.

@DrZ214: Replace spark plug with injection nozzle, and Diesels have similar problems. Here the time to heat up all the gas volume by combustion is equally dependent on cylinder size. As with gasoline engines which have more than one spark plug, adding more injection nozzles helps. Regarding the 2-stroke process: This thing runs extremely slowly and has plenty of time to inhale some fresh air when the piston is down. Much of the gas in the cylinder is not replaced and becomes a process gas in the next cycle. This is the simplest way of exhaust gas recirculation possible.

@DrZ214: The two-stroke process gives you twice the number of ignitions at the same speed, so the engine is used more economically (instead of idly running pistons up and down half the time). The Jumo 205 Diesel engines were two-strokes, too, as was the Napier Deltic. Make sure to read up on the Deltic, this was one clever contraption!

@DrZ214 2-stroke marine engines are diesels. They don't have a fuel intake stroke, it's just air intake (fuel is injected after compression). So fuel can't escape via the exhaust, unlike a 2-stroke petrol engine. That's why 2-stroke is a viable option for diesels, and halving the number of cylinders is a win...

@PeterKämpf Here the time to heat up all the gas volume by combustion is equally dependent on cylinder size. Right but my point was that the fuel/air mixture detonates all at once because the charge reaches adiabatically auto-ignition conditions simultaneously. So there is not speed of flame front. Correct me if I'm wrong but you are concerned with 2 things: injecting and mixing the fuel so that the fuel/air charge is uniform, and the travel length of the piston and its arm. Those are certainly two things that still apply to diesels.

@PeterKämpf P.S., your graph is hard to interpret. Why is it putting individual engines on the x axis? Why would there be some horizontal relationship between different engines, which as far as I can tell are just arbitrary selection of engines? I think it should be a table.

@DrZ214: Fuel is injected in Diesels long after the auto-ignition conditions have been reached - otherwise no complicated high-pressure fuel injection would be needed. Fuel vapor spreads out from the nozzle and ignites at the spray penetration boundary after an ignition delay period caused by the heating of the fuel vapor. This is not an instantaneous, sudden ignition of all the fuel but a cone originating from the injection nozzle in which the outer parts ignite first and heat up the rest which burns as it mixes. See here for details.

@PeterKämpf The link says it's done right before auto-ignition conditions, not after. Either way, it's odd. Don't see why diesel injection is done the same way as avgas injection. For that matter, why not just pre-inject fuel such that the valves suck in or inject a premixed fuel/air charge? This is proly getting too off-topic, but I will read any other links you have on the diesel cycle.

I do not think you can posit that stronger materials could compensate for higher loads because surely the best feasible strength / weight materials available would be used in either version. Consequently, the fewer-larger cylindered engine, with its higher peak loads, would not necessarily have a weight advantage or even parity - it would have fewer but heavier connecting rods, fewer but longer big-end bearings etc.

@DrZ214: Technically, auto-ignition conditions need both the right temperature and the right fuel-air mixture, so they can only be reached after injection. What I meant is that the compression-induced temperature is high enough before fuel is injected, or the high pressure and exact timing would not be needed. Gasoline injection is a comparable lazy process happening in the suction pipe or during the compression stroke, and gasoline compression ratios are limited by the knock limits. Diesels operate far beyond the knock limit.

@sdenham: Isn't it strange then that in many cases speeds could be increased by using stronger connecting rods? In reality, the most economical material was used, not the best. Welcome to reality!

Alloys such as 4340 steel are not chosen over cast iron for economy.

Others have already mentioned the scaling of volume vs scaling of surface. However the most important part about the surface is valve area.
When you scale a cylinder 2 times you get 8× as much volume but only 4× bigger valves. This means that same volume of the cylinder is now served by 2 times smaller valve area. This area determines how fast you can fill and empty the cylinder. This means that you have to turn the rpm down. As more rpm means more power, that means you get diminishing returns: twice as big cylinder will deliver less than twice as much power.
Adding another cylinder, on the other hand, is almost perfectly linear: twice the cylinders mean twice more power.

You said it yourself with the surface-area-to-volume-ratio. You have to get the heat out of the cylinders, and if they're too big you can't do that effectively.  It's also difficult to get even, complete, rapid combustion as the volume increases.

I don't think this reasoning is correct. Engines don't need cooling of the burning air/fuel charge, they need cooling of the cylinder wall and the oil film on the wall to prevent it from decomposing. For efficiency you want the charge to be as hot as possible, hot air exerts more pressure on the piston. Less heat loss to the cylinder wall is one reason why large cylinders are more efficient than small ones. Having said that, larger cylinders require thicker walls to withstand the higher total forces and that may be an issue for air cooling.

Most was already said, but I'd add following:

More displacement means heavier pistons which have higher inertia. This limits RPM and produces serious loads on engine components. To withstand these loads, other components have to be more rigid and thus heavier.
Power is the product of momentum and revolutions. Increasing rpm rate
yields power faster and it is easier way to get more power (to a certain extent) rather than increasing momentum. To rise RPM,
lighter internal parts should be employed. AFAIK, in aircraft
applications, unlike automotive, higher RPM is preferred over higher momentum. You don't need power at the low-rpm end as much as in a car.
The more displacement one cylinder has, the more difficult it is to
achieve uniform mixture formation and effective, complete combustion. That is
why in automotive engine 4-cylinder engines most often limited to
2.0-2.5 liters, 6-cylinder - to 3-3.3L, 8 cylinder - up to 4-5 liters, and so on. This keeps volume per cylinder to a certain reasonable level (0.5l/cylinder).
Limit of volume per cylinder is also determined by combustion speed. On high RPM it may turn out that combustion is not finished when power stroke is complete, so flame shoots up from cylinders and eventually melt valves. As a variant, the engine won't be able to speed up over certain RPM at all. This issue could be partially negated by early ignition and double spark plugs, but again this is not as effective as keeping good volume/cylinder ratio.

@Federico Thanks for corrections!

With more cylinders, the strokes are so timed that, when one cylinder is compressing, some other gives out power, and so on. This ensures that the power output (or mean torque, as will be shown in a T-theta diagram) will remain constant over complete rotation of crank.
Kinetic energy stored in flywheel is proportional to it's mass (actually mass moment of inertia). If engine requires less energy from flywheel for compression strokes, K.E. required to be stored in flywheel is less. And flywheel could be made lighter .

Could you provide such a T-theta diagram? Most people probably don't know it.

Do aircraft engines really need a flywheel? I would have thought the propeller could serve that function. And anyway, with 6 or more cylinders, the inert strokes should be pushed through by the powered strokes so I'm not sure if a flywheel is needed to balance it out.

Planes need to conserve weight. In other words: it wants an engine with a high power/weight ratio. The power produced by a cylinder is proportional to the surface area of the piston (if the pressure stays the same). So, if you divide all the dimensions of a engine cylinder by 2 the power produced is 4x smaller, but the weight of the cylinder is 8x smaller. Hence, the power/weight ratio is twice as high.
That's why planes prefer engines with lots of small cylinders over an engine with a few large cylinders. In engineering this is called 'dimensional analysis', see https://en.wikipedia.org/wiki/Dimensional_analysis

Strictly, the force produced by a cylinder is proportional to the surface area  of the piston if the surface area stays the same. To say that the power (horsepower or kW)  produced by a cylinder is proportional to surface area we must add the assumption that the average velocity of the piston remains the same. Broadly, this assumption is correct and means that that the longer the stroke of a piston, the lower the RPM of the engine, which hurts power/weight ratio as you describe.

I don't understand your reasoning. Work by the piston = Force x Distance.

And power = work / time = force x velocity.

I don't understand '
Strictly, the force produced by a cylinder is proportional to the surface area of the piston if the surface area stays the same'

You said the POWER produced by the cylinder is proportional to the surface area of the piston if the pressure stays the same. I corrected this to the FORCE produced by the produced by a cylinder is proportional to the surface area of the piston is the surface area of the pistion if the pressure stays the same. Force = pressure x area. Power = work / time = force x velocity = pressure x area x velocity.

Ah, ok. But there is no need to (re-)introduce the time factor. Another example: muscular power is proportional to the cross area size. When you grow from 5 to 25 years, you are twice the size. Hence, your muscle power is 4x it  was, but your weight is 8 times it was. That's why you now have trouble walking with a same-size-friend on your back, but it used to be simple.

muscular FORCE (measured in newtons, lbf or kgf) is proportional to muscle area. muscular WORK is given by force times distance. Therefore to lift your friend when you are taller, you have to lift him twice as far, which is another, different way of looking at why it is more difficult. POWER is work / time. So if you run upstairs you generate more power than if you walk upstairs. When talking about the relationship between force and power (in the physics / engineering definition, which may be abused in general conversation) it is always necessary to mention both time and distance.

Im a mechanical engineer, I know the difference. What I tried to show is, that both in the engineering meaning of power, and the general meaning, the effects are the same. The force dubbles, so does the power. You can talk about force, work or power, the effects are the same.

According to Wikipedia, which you cite,  dimensional analysis is the analysis of the relationships between different physical quantities by identifying their base quantities (such as length, mass, time, and electric charge) and units of measure (such as miles vs. kilometers, or pounds vs. kilograms vs. grams). This does not cover the case you describe where changes on one dimension produce disproportionate changes in another.

Early aviation was not based on very much on all these scientific or engineering concepts, but based on what they found worked.  Many early aviation engine manufacturers primarily came from automotive industry, and they took what they knew worked, and doubled it to meet power requirements (flat 6 to v12).  Why they tended to not simplify and reduce the number of cylinders probably had a lot to do with reliability (more cylinders, more redundancies).  The British, and by alliance Americans, had the first jet engine concepts of the war but focused on more practical technology; which would have you wanted to test fly?

On the contrary - the Wright Bros were the first to develop an accurate and reliable science of aerodynamics, which is what allowed them to develop a working airplane.  Aviation has always been based around science but is reluctant to stray too far from established and workable designs

In addition to what has already been said here, there is a piston speed limit, high revving engines have shorter crank throws. The flame front and thus the piston can only travel so fast, huge engines like boats have huge throws, low rpm engines, but have similar maximum piston speeds to car engines.
Why not make the crank throw the same but have really large diameter pistons? well that can be done, but only up to a point, too wide and the crank shaft gets too weak and the engine becomes really wide.


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Who was the first black person (in the world) to go airborne in an aircraft?
Who was the first black person in the world to go airborne in an aircraft?
Surprisingly, google fails me in this so I don't think it falls under the closeable category of trivial trivia.
(I do not use the term "African American" because the answer might not be an American. I don't use "African" either because there are white Arab Africans. I suppose Sub-saharan African? But then there are black Africans on the north coast of Africa too. So I don't know what the term is that's both accurate and politically correct.)
Closest I've found so far:

Bessie Coleman was the first black woman to hold a civil pilot license, but I could not find a date for her very first airborne event, either as a passenger or pilot.
Eugene Bullard was one of the first black pilots, as was Ahmet Ali Çelikten and Emory C. Malick according to wikipedia. I could not find dates for their very first airborne events either.

My question is about any such person to be airborne in an aircraft. Obviously this could be a passenger, not a pilot. Edit: and it could be an employee too. I don't know how the personnel was organized back then, but maybe some cargo handler went along with some flights.
BTW the definition I'm using for "aircraft" is any heavier-than-air thing flies via lift. So no hot-air balloons and no dirigibles. But rotary-aircraft and gliders are okay. I think they had experimental ones very early in aviation history.

It seems much more likely to me that the first black person to fly would have been a passenger rather than a pilot. I suspect also that the first black person to fly would have been an African in Africa, in a European colonial possession (i.e. nearly all of Africa in the first decade of the twentieth century), and almost certainly as an employee rather than a paying passenger. But that's only a guess.

It seems related to segregation or something like this and you are likely asking for the US. There are countries where the only pilots and passengers are black and where the question is not really pertinent. Maybe you want to explain a bit more and use the tag "usa". Also the French pioneers in airmail used to fly over Africa and were assisted by local interpreters in case of forced landing in the desert or rebels areas (asking for ransom was usual at this time over 1917-18)

Ahmet Ali Çelikten seems to have been of mixed race, mostly Turkish, not black.

@mins No, I want to know the first black to fly in the world. Of course it's still related to segregation because aviation was invented in 1903, a time when there was much racism and segregation all over the world, not just in usa. I edited the OP.

"there was much racism and segregation all over the world, not just in usa": Actually there was few countries with segregation: US, South Africa, Australia, Germany (briefly against Jews), Rhodesia and Malaysia. Segregation against Black people occurred only in US and South Africa. So your question is basically pertinent for US and RSA. Black people not flying early in other countries is because of other reasons.

Isn't this question off-topic?

You're spending so much time trying to show you're not racist by inventing terms to replace "black" or "negroid" that it's silly. Except some people in the US nobody cares about the term "black" or "negroid" in general, are smart enough to spot how it's used in context and whether than context is deregatory or not.

Since you specified "aircraft" you would have to consider the aircraft category/class of "lighter-than-air/balloons" which date back to 1783.  It appears most answers have limited their response to considering only the category "airplane".

@Gerry I think you overlooked my final paragraph.

The earliest example I could find is Emory Malick, a African American Aviator who received his pilots license in 1912 but stopped flying after a series of crashes. For the first pilot with a major career, every source I could find points to Eugene Bullard, an American who studied in France and fought in WWI, receiving his license in 1917.
Specific searches to look for British, German, or Italian pilots of African descent came up with nothing from that time period, so I think it's fairly safe to say that these two were the first.
Outside of the US, Ahmet Ali Çelikten flew for the Ottoman Empire beginning in 1914, and is widely regarded as the second black pilot following Malick.
As for passengers, I doubt you'll find much unless logs from the time included race on tickets for aviation.

I understand that the US did not issue pilots licenses until 1927. Where did these early pilots get the licenses you mention?

It's in the article. The Malick and Bullard articles both specify license numbers in them. Bullard got his in France, and it isn't said for Malick

Yes, the Malick article mentions licenses. What it doesn't note is that these were not Government issued licenses. Clubs offered licenses. See the article comments.

"As for passengers, I doubt you'll find much unless logs from the time included race on tickets for aviation." Maybe, maybe not. Back in the day, passenger tickets were pretty expensive, so whoever bought them proly has plenty of other records associated with his/her name. It would require some serious sleuthing, however.

I think your assertion about the US being the first to have black pilots is probably wrong and certainly naive. Other countries tend to not make distinction based on skin colour when issuing pilot licenses, and never have (with few exceptions like potentially the RSA).

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open windshield during landing/takeoff
In India, during landing and takeoff the passangers are asked to open the windshield.
I googled the reason but couldn't find any technical logic.
I am also not sure if this practice is universal.
Can anyone explain reason.

It is a matter of safety. In case of an accident, being able to see out the windows will make it easier for crew and passengers to orient themselves, for example to spot a fire on one side of the aircraft.

Agree...This is duplicate...Auto suggestions didn't work in android app hence couldn't notice it.

This is not the windshield that is open, but the windows shades.

Yes, the windshield is the thing at the front of the airplane the pilots look through.  Some military planes have anti-flash curtains (you pull those shut in cruise when flying on instruments and nukes are flying; it's pretty dumb to be blinded by a distant H-bomb flash that couldn't harm you otherwise).  But you're supposed to have those open on takeoff, they really hammered that home in training :)

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Can airliners fly inverted?

If an aircraft flies with it's belly facing the earth (or "Blue side up"), the how can it fly around the world without flying inverted?

Possible duplicate of Can large airliners do aerobatics such as loops? You might also want to ask some questions on physics.SE about how gravity works.

Because your frame of reference moves with you.

what is a "frame of reference"?

Possible duplicate of Do pilots adjust the aircraft's flight path to allow for the curvature of the Earth?

@thales727 This is much better suited to a physics site. A frame of reference is a relative coordinate system. Just like "up" is the same in antartica as it is in the north pole, the difference is your frame of reference. In the image above, the FoR is something in space, like the moon. If you were in the aircraft, the FoR would be your place in the plane. By the way, FL430 is not considered "extreme low earth orbit" just like 8 inches off the ground isn't "ultimate low earth orbit".

Use the centre of the Earth as your frame of reference. Gravity always acts (to all practical purposes) towards the centre of the Earth. Therefore, the "belly" of the aircraft is always pointing down. You can get many good answers here, it's simply true that you will get more, and better ones, at Physics.SE since this is a question of physics, and not aerodynamics. Ron Beyers answer is a good one. The frame of reference you are using it outside the system

As others have mentioned this is a classic frame of reference problem in physics, perhaps second only to the Einstein on a skateboard joke (Professor Einstein enters the classroom on a skateboard and asks who is moving - the students declare that he is, and he responds that from his perspective he's standing still and they're all moving around him before launching into a discussion of special relativity.) See also: Bob Hoover pouring iced tea upside down during a roll.


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How autonomous are commercial passenger airliners?
Average layman here. Assuming the latest generation of planes/avionics rolling out of the factory today, how autonomous are commercial passenger jets? Obviously, autopilot has been around for decades, but what are the limits of its capabilities? Can it take off and land by itself? What can be done remotely, if anything?
Watching a typical airliner-crew-is-dead-what-can-we-do-now movie caused me to think of this question.

Hello iMerchant and welcome to aviation.SE. Your question seems too broad to me. What are you asking? If the autopilot has taken up a significant amount of pilot duties? If a passenger plane can be remotely controlled? What happens if both pilots are incapacitated?

You may be interested in the autoland tag.

See also Has there ever been a fully automated takeoff, cruise, and landing of a large aircraft similar to commercial airliners? and How does auto landing work?

Voting to keep open, mainly because this question (and the answers) focus on the difference between automated and autonomous.  The other questions focus somewhat more narrowly on specific autopilot capabilities, while this question looks at the broader issue.

They're not really autonomous, they're mostly automated.
I'll lift this description from Wikipedia despite being written for cars (emphasis mine):

Autonomous means self-governance. Many historical projects related to vehicle autonomy have been automated (made to be automatic) due to a heavy reliance on artificial hints in their environment, such as magnetic strips. Autonomous control implies satisfactory performance under significant uncertainties in the environment and the ability to compensate for system failures without external intervention.

While most autopilots will fare fairly well under "significant uncertainties in the environment" (read e.g. "severe turbulence"), they are not equally capable "to compensate for system failures without external intervention". Even the Airbus' autopilots revert authority to the pilots if the system degrades.
Even knowing the destination airport, they need a pilot to insert the flight plan into the flight computer, they cannot compute it by themselves.
They cannot take evasive action in case of dangerous situations, but they can only warn the pilot (see EGPWS and TCAS RAs).
They cannot even properly recognize bad weather (say, a thunderstorm) and eventually plan diversions around it.
They could not land an aircraft by themselves, as they generally lack control over the aircraft configuration.
What they can do is blindly follow the flight plan given, so they are automatic, but not autonomous.

Good stuff but I disagree on the landing comment. Auto-land capabilities  has been around since the days of the Concorde

@Dave that's still automatic, not autonomous: the autopilot cannot negotiate a landing clearance, it cannot initiate a go-around in case of emergency, it cannot lower flaps and gear, it cannot select an alternate airport in case of need (and cannot recognize the case of need), it cannot land without the ILS antenna, i.e. it relies on artificial hints in their environment. It is automatic, not autonomous.

I will agree with that but strictly speaking they can land by them selves if set up properly

@Dave "if set up properly". That's the point. It's not autonomous if it needs "set up", and if it is not autonomous, is not strictly "by themselves".

I think the distinction here between autonomous and automatic is a little arbitrary. The case in point being that TCAS has been integrated into AP already (e.g. the A380).  Other feats like one-engine-out compensation, complex RNAV approaches, pitch-up overspeed protection, windshear modes, radios that identify the station, and emergency descent modes would be seen 60 years ago as amazing feats of computer decision-making.

@CodyP the TCAS integration is currently the exception, not the rule, and I wrote "mostly". And all those things you mention are pre-programmed actions, the machine is not determining a new course of action by itself. What was 60 years ago does not change the fact that today the scientific community defines "autonomous" something that has the answer "yes" to the question "Can this machine complete any full mission, with all possible emergency scenarios, without any human intervention except switching it on?" If not it's either only partially autonomous or simply automatic.

Even the most modern aircraft autopilots of today are unable to be "autonomous".
The aircraft can not be programmed to takeoff or land by itself. Auto landings are possible, but only with inputs by the pilot.
The routing can be data linked to the aircraft, but it still needs the pilot to press the button to load it into the flight computer. After takeoff the A/P can follow the programmed route, but it is unable to climb or descend without input from the pilot. Any change in the routing (like avoiding thunderstorms) must be manually inputted by the pilot.
Currently, nothing can currently be done remotely. Remote control could be designed into an airliner, but it just hasn't been done yet, and probably never will.


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What are the names of the vehicles to provide assistance to aircraft during the turnaround?
During the turnaround many vehicles support the aircraft on the gate. What are the name of those "vehicles"and their purpose?

there are a lot of different vehicles, list style questions like this are not a good fit for SE in general.

In my experience, almost all of them are referred to as "tugs", since they are usually pulling or pushing something (luggage carts, the aircraft, etc).

I wouldn't say that @RonBeyer, there's follow-me cars, catering lifts, luggage ramps, etc. I agree this is a bit broad.

Fuel trucks, food trucks, snow plows...

@GdD Yeah, forgot about those ones. VTC though for being too broad.

Try this diagram

I was about to post an answer with this information which provides names, but I see @TomMcW already provided a best one. See also on Wikipedia: Ground support equipment. You may also search videos for "turnaround timelapse". Come on, this question isn't "too broad" and it's actually a very interesting one.

The most common vehicle seen is the tug.

source

source
Tugs are multipurpose tractors that are used to tow around all sorts of things.

Several types of equipment are shown in this diagram:

source

The tow vehicle at the nose is more commonly referred to as a pushback tug. They come in different sizes. Here is an example:

source

The GPU is most often a unit that is towed behind a tug.

source

Belt loaders usually can be driven on their own:

source

The wide loader is used to put cargo containers in the hold:

source

The cabin cleaning truck and galley service truck often have a lift built in:

source

Luggage can be brought in baggage carts

source
or in cargo containers which are brought to the aircraft on dollies

source

Another vehicle often seen is the people mover:

source

In general, you could refer to most of the vehicles and their attachments/trailers as ground support equipment (GSE)

This does answer half of the question only. What is their purpose?

The vehicle is usually called a ground tug.

There are many other vehicle types used too.

Turnaround does not refer to physically turning the plane around ;)


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What is this old US Army Air Corps plane?
This photo was taken by my Grandfather in 1918 at Rich Field in Wako, TX.  I don't think that it is a Taube because of the front structure, engine type, 4 wheels, fuselage shape, etc.

Welcome to aviation.SE! This type of question is definitely welcome here and I hope someone can identify it. I rearranged your question slightly but if I've changed it too much please feel free to roll back my edit or re-edit it yourself.

@PerlDuck Thanks. Perhaps. Spelling corrected.  (Dreaming of wine, I guess.)

Well gosh, this is a rare bird! The 'DEHAN' monoplane:

From this forum:

More about Dr.Ing. Jacob Emil Noeggerath (Nöggerath ?),from the first
  Lange's book :
He had in 19011/12 the HUNGARIAN " DEHAN ":) monoplane designed ,wich
  was flown at München-Oberwiesenfeld.
There is another short passage about it in the Etrich book by Hanuš
  Salz.
Dr. med. Theodor Dieterle let built this monoplane in the Otto works
  in Munich in 1913. He was supported by J.E. Noeggerath (not Nöggerath)
  who corresponds with Etrich since 1911 and was probably involved in
  the design too.
It had an unusual longitudinal stabilisation system with a sort of
  tank? at the front that later was removed.
Hanuš Salz named it "Dieterle Schwalbe" or "Taube" only. I prefer the
  term "Schwalbe" as the wings are peaked and the tail even it is not
  splitted but inward curved and not Taube-like.
After this machine was tested with the swiss army, they contacted
  Etrich as well. Only an order for his machines was not realised.

So, it would appear that the plane was a one-off, commissioned by two medical doctors in Zurich, designed by Etrich (hence the similarity of the wings to those of the Taube), built in Munich and flown in Switzerland.
Which does, of course, leave another question: how did it turn-up in Texas in 1918? (Or did it?)
Edit:
There is quite a lot more info on this page under the name 'Dieterle Schwalbe'. In particular:

Als Besonderheit verfügte die ‚Dieterle-Schwalbe‘ vor der Rumpfnase über eine mit Bleischrot gefüllte, jederzeit verstellbare Metallkapsel von ca. 10 kg. Mit diesem Vorgewicht konnte der Aeroplan während des Fluges ausbalanciert werden. Mit der Lenkbarkeit und dem Sturzflugverhalten von Flugapparaten hat Dieterle sich intensiv beschäftigt und seine Erkenntnisse in mehreren Bulletins des Aero Clubs 1914/15 veröffentlicht. Ob das Resultat seiner Forschungsarbeit in die Entwicklung späterer Sturzkampfbomber wie der Ju 87 eingeflossen ist, kann vermutet werden.

Google translation:

As a special ordered that, Dieterle Swallow 'in front of the fuselage nose over a column filled with lead shot, at any time adjustable metal capsule of about 10 kg. With this pre-Wicht of Aeroplan could be balanced in flight. With the steerability and the nosedive of Aerial equipment Dieterle has intensively and published his findings in several bulletins of the Aeroclub 1914/15. Whether the result of his research has gone into the development of later dive bombers like the Ju 87, can be suspected.

In other words, Dieterle specially ordered that there be 'pre-weight' canister of about 10kg lead shot attached forward of the fuselage and which could be adjusted in flight to balance the aircraft.

Caption reads: "The 'Dieterle Swallow' with the characteristic Pre-weight; Oberwiesenfeld January 1913"

Thanks so much for finding this.  My question is answered.  Well, that ended 60 years of speculation.

Regarding the location of the plane, they were in an album of WWI era photos.  My Grandfather served in the US Army Air Service.  Here  is a picture of him from that album.  The back is annotated "March 10, '18 Slocum & I at Anderson's farm, Speegleville, Tex."  It was from that annotation that I assumed the airstrip was Rich Field.  Here are the other plane pictures in the album.   I'd also like the 2nd and 3rd planes identified but I assume that I will make separate posts for those?

The second aircraft is almost certainly the (unsuccessful) Wullschleger-Peier Triplane ..another Swiss aircraft of 1913!

Backstroke!  I checked with the family.  My Grandfather was in both Munich and Switzerland before the War.  He was an engineer so I expect that he sought out these planes.  He probably just grouped his plane pictures together.

Thanks again so much for identifying the second aircraft.

Standing ovation required!


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How does type ratings training work at the airlines?
How does it work to get typed in a different aircraft at a carrier like United, American, and/or Delta? If you're a first officer on the A320, but want to be a first officer on a different aircraft? Is it seniority and bidding on a different aircraft? How long would that typically take to get to that point to change aircraft?
Getting ready to start at a regional, and was curious. I'm also wondering how you get selected for the aircraft you will fly once reaching a major

The carrier trains you for their aircraft.  If you don't already have a type rating for your new aircraft, then you'll get one as part of the checkride at the completion of Captain training. (Maybe you flew that aircraft at another carrier, or you were an International Relief Office -- a type-rated F.O., or maybe you simply got the type rating as part of F.O. training on that aircraft earlier.)  If the carrier needs you to have the type rating on your new aircraft as an FO, you'll get that as part of your training for that aircraft. If you don't need it, you probably won't.
Once you are in the system, everything is driven by seniority -- what do you have the seniority to hold -- and the rules of the contract.
Each fleet at each carrier, and more often than not, each domicile at each carrier, will have different dynamics.  In some cases, pilots "camp out" in one aircraft at a particular base for a long time; in other cases, after a couple of years, they're typically bidding (exercising seniority) to go back to training for the next step up the ladder (MD80 to 737 to 757 to 777 FO, then MD80 Captain to 737 to 757 to 777).  Different contracts have different rules; some allow a pilot to camp out as long as he wants -- perhaps he's enjoying the quality of life being a senior FO on a nice aircraft at a base close to home, and he isn't interested in upgrading to a bigger aircraft (where he'd be less senior) or the left seat of a smaller aircraft (where he'd also be rather junior) at a base far away. Other contracts (historically) have had provisions that once you can hold either your same seat on a bigger aircraft (or the left seat on any aircraft for an FO), your clock starts & within so many months, you have to upgrade (assuming one is still available) -- for whatever reason that contract was written to avoid having lots of pilots camping out in lieu of upgrading.  All very dependent on the contract in place where you work.
Also, the state of the economy and of the airline can drive a lot of this... with Delta retiring many, many senior captains over the next few years, there are lots of opportunities for progression.  When everything tanked after 9/11, forward progress stopped and in many cases people moved backwards... MD80 captains went back to being FO's, widebody FO's went back to being narrowbody FO's, junior FO's were furloughed, etc.  During the stagnation of the 2008+ recession coupled with the change to Age 65 retirements, nobody upgraded due to retirements (okay, very few retirements), and almost nobody was growing their fleets, so pilots stayed put much longer than they might have otherwise -- there was nothing available to bid FOR.  You can only exercise your seniority for a slot that is open.

I'm also wondering how you get selected for the aircraft you will fly once reaching a major

It's essentially whatever aircraft/base needs the bodies that month.  One class may get a lot of 737's and a few A320's, another class gets all 767's.  Often, the vacancies in the 767 would be filled by pilots moving up from the smaller aircraft, but that doesn't always occur, and it's not at all unheard of for newhire FO's to sometimes go to much larger (read, better-paying) aircraft.  Of course, they'll be junior on that aircraft for a long time; they'd get a "more senior" schedule -- fewer weekends & holidays at work -- sooner on the smaller, more junior aircraft.
Likewise the domicile -- if JFK has the openings, you go to JFK (I'm so sorry). If JFK is full & ATL has the openings, you go to ATL (which is probably better, unless you live close to New York).  Easier to change domicile than to change aircraft, since no retraining is typically required. (Maybe an "international" course if your old domicile flew only domestic and your new one flies international -- but that's no big deal.)
Welcome to the airline industry!

Thanks a lot! I'm excited to finally get going. Been toughing it out as a CFI. Great information!

Glad I could help!

International carriers (such as the Middle East Big 3 - Emirates, Etihad and Qatar) work a bit differently.  You get different perks, the line of succession is clearly defined and in most markets there are no unions. For Emirates you'll always be a FO on a widebody jet and flying international routes (which are lucrative in other markets).


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What is the optimal time to get from ASEL to ASES certification?
I am currently a ASEL pilot, in process of studying for IR, and will then be proceeding to AMEL. I know that if one holds ASEL & IR, and takes AMEL with instrument maneuvers included in the checkride, at the end I will have IR privileges in both single and multi.
What is the most optimal (time & cost) route to get my ASES? Are there any efficiencies to be had by completing seaplane rating after IR and/or multi? Or if I was to complete ASES before IR, once IR is done, I could exercise IR privileges when operating a seaplane (unlikely scenario, just want to understand the rules). From my understanding, I will, because IR applies to the category of airplane. Is my understanding correct?

Interesting question -- welcome to Av.SE!

The instrument airplane PTS has both maneuvers for single and multi engine airplanes. Obviously, when you first get your IR in an ASEL airplane you cannot complete the maneuvers required for multi-engine airplane.  Hence, if you choose not complete those maneuvers when you take your AMEL checkride, you will be limited to VFR.
There are no specific IR maneuvers in a seaplane so once you complete your IR in a ASEL airplane, you will also have IR privileges in a ASES airplane.
The answer is found in the instrument PTS / ACS book.
If you were to have both ASEL and AMEL ratings and took your initial IR in a AMEL, it would be valid for all aircraft that you are rated in or get rated in later on: ASEL, ASES, AMEL, AMES.


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How do small planes cross oceans?
I've often flown short flights around the US on aircraft built by non-American companies whose range is less than is required to make a traditional Atlantic crossing.  The Airbus A319-100, for instance, has a range of 2,400 miles while the distance from London to NYC is 3,440 miles.
How do manufacturers get their small planes across oceans?  Do they operate factories in on all continents? Do they fly via Iceland or Greenland?

The actual duplicate is How do airliners get from the factory to the client if the aircraft does not have the required range

@DeltaLima Actually, in this case, it does have the required range. You just make a fuel stop in Iceland. Also, with the A320 family, your plane might not actually be coming from France, but rather from Alabama.

@Reirab Isn't the fuelstop in Iceland a result of the lack of range to reach the customer? Anyway, the A320 can reach the USA directly from the factory in Hamburg or Toulouse, albeit on fumes. The BA1 A318 service from London City airport to New York makes a fuel stop in Shannon (Ireland), but takes 32 business class passengers with it and crosses on a daily basis.

@DeltaLima I was taking "doesn't have the required range" in the linked question to mean "the route contains at least one leg that the aircraft cannot fly with its normal range," like in the case of the Hawaiian Airlines 717s mentioned in the first answer. You're right, though, that the A320 family does normally have more range than what the OP linked for the Delta A319s. Wiki lists the A318 at 3,100 nmi at MTOW (though that's still just barely enough for BA1, especially going against the wind towards the U.S.; I'd assume it flies at least somewhat light.)


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Is it allowed to operate an aircraft when the Emergency Locator Transmitter has been removed?
I purchased an aircraft from which the previous owner removed the ELT before I took possession; it was present during the test flight. I need to know what the minimum legal requirements are for a replacement. Can I use a personal unit which are sold everywhere or do I need a specific unit for general aviation?

What kind of aircraft?  Experimental?  How many seats?

Did you buy an A380 or a 152? That can be a big difference.

As a side note, was the plane to be sold "as is"? If so, I'd think that would include the ELT and that you should go asking the previous owner to give it to you. Unless, of course, you knew it was present only for the test flight and would be removed again afterwards.

Assuming that you are in the US, you need to comply with
§91.207   Emergency locator transmitters.
(2) … there must be attached to the airplane an approved personal type or an approved automatic type emergency locator transmitter that is in operable condition,
and
(ii) No person may operate the aircraft more than 90 days after the ELT is initially removed from the aircraft; and
So in the next 90 days you need to install an ELT. It must meet the TSO standards for ELTs.
The cheapest way to comply would be to purchase a used 121.5 MHz unit so you are legal and use a personal unit in case of an accident.
The newer 406MHz units require a GPS source and a panel switch. Installation costs can be more than the unit. However, since the previous owner removed the ELT, I would assume that the wiring is still intact so you can probably save money on the install.
Which type of attached transmitter you choose depends on your budget and the terrain over which you fly. On February 1, 2009, the international COSPAS-SARSAT satellite system discontinued satellite-based monitoring of the 121.5/243-MHz frequencies, in part because of a high number of false signals attributed with these frequencies. AOPA
They also frequently fail to activate and since they are only monitored by pilots who are following the AIM and monitoring 121.5 even if they do go off, they often are not heard. Not to mention the problem of triangulating the signal to find the source. If you fly over populated areas and are in contact with ATC via flight following or IFR flight plans, they are still adequate and most pilots I know have not upgraded to the 406 MHz units.
The newer 406 MHz are satellite monitored and because they transmit GPS location data it is easier to find a downed aircraft. They have an activation switch in the panel so that they can be activated before the pilot hits terrain which increases the odds of finding you. If I frequently flew over mountains, remote areas, or at night I would certainly install one of these.
Personal beacons have lots of advantages over installed ELTs. If you survive the landing, chances are that they will as well. They are also incredibly cheap and many of them have web-tracking or texting that can be used by those interested in tracking your progress. You can also take them with you if you leave the airplane.


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What is a safe but non-alarming proximity distance setting for SafeSky?
I am using an app for General Aviation called Safesky. It works on cellular data, is free and provides a lot of the functionality that a dedicated box like Pilot Aware does.
However, I have set my range to 5NM and 2,500FT. Because I fly in busy airspace, this results in many proximity warnings, which causes extreme fatigue for me personally.
My logic for this setting, is that within 5NM, another aircraft could descend 2,500FT onto my level, but other more experienced pilots said this is not a distance to worry about.
What would be a safe proximity setting in terms of vertical feet and horizontal distance for proximity alerts in GA?

Please don't just provide numbers as an answer, but also provide the reasoning, facts and principles that lead to these numbers.

Very opinion based. OOI, which airfield do you fly from (if you're comfortable sharing that info)? I'm regularly in that section sandwiched between Luton/stansted and Heathrow/city (which is purported to be the busiest GA airspace in europe!) and I get traffic notifications within about 10NM, it's never bothered me too much. But that's because its just a backup for a good lookout. I know when I can just ignore it.

More in terms of answering your question, as im not that familiar with SafeSky - how do you get notifications from it? SkyDemon? Audio-only? Dedicated app on your phone screen?

Voting to close because of the opinion-based nature of the question. This is a good question for a pilot forum, though, and, personally, I am interested to know what the consensus is.

Don't agree that this is a purely opinion-based question. Answers may contain a level of opinion, but as long as they are supported by facts and principles, such opinion-based answers are valuable.

@Jamiec SafeSky is a mobile app that runs as a dedicated app on your phone. You can also run it as a split screen app on your tablet and it can stream it's data to/from your navigation app or other traffic related systems. https://www.safesky.app/ . Note: I'm not affiliated with SafeSky, just a happy user.

@DeltaLima Maybe not purely, but almost exclusively. For a start it's going to depend a lot on the airspace you're in, and the speed you're travelling. I know the answer to 1 of those but they should at very least be part of the question - otherwise it is absolutely opinions only.

@Jamiec, I am writing up an answer that gives a methodology to arrive at a reasonable number and allows for adjusting based on traffic environment and risk-appetite.

@Jamiec You can integrate SafeSky with SkyDemon (with a premium plan), it gives you audible and visual warnings, including a terrifying 'time to impact' countdown timer... hence my stress levels :D Flying at a collection of busy Microlight sights, I won't share specific airfield info, although I can tell you cruising at around 70kts usually

If you're concentrating on a "Time to impact" counter, then you're using electronic conspicuity  devices all wrong! Eyes outside.

A relevant factor may be how many unequipped aircraft may be likely to present, none of which you'll detect if your eyes are mostly inside the cockpit!  Ditto for large birds for that matter...

@DeltaLima I don't see how this can't be opinion based. Your answer, as well argued as it is, is opinion-based, based on suppositions and feel-goods. It's akin to asking someone when to flare. There are lots of good answers, and they're pretty much all right. Some people do it with a formula and others do it by feel, but there is no general answer. Maybe if the question narrows down, e.g. to pull from my example "when does the FAA teach to flare" instead of "when should I flare?" Otherwise, this is great stuff, but it's forum stuff not Q&A fodder.

@KennSebesta I agree that the answer is to a certain degree opinion-based and I don't think there is any problem with that. This discussion is as old as this site, and pre-dates it in the stackexchange ecosystem. The off-topics guidance in the help centre for this site states: You should only ask questions that can be factually answered or seek opinions that are supported by facts and principles.. Not every question that is opinion-based is problematic for this site.

I don't know how you arrived at the 5 NM / 2500 ft numbers, but it sounds like a plausible ratio. However, what is really important is the distance magnitude here. 5NM / 2500 ft is the same ratio as 10NM / 5000ft or 2 NM/1000ft. The former is giving you many more nuisance alerts than the latter, but the latter will be giving you less time to react.
What you have here is the typically challenge that every designer of a safety relevant alerting system is always faced with: a trade-off between the number of false alerts (nuicance) and the number of missed alerts (dangerous).
To arrive at a good distance and vertical settings, you would need to look at 2 things:

what is the time that is needed to react to an alert and be able to execute an avoidance manoeuvre, and
what is the relative speed that the two aircraft move at?

For the first part, let's look at the timeline of events leading to a safe outcome. I don't only look at the pilot's reaction to the alert here, I also look at the time to generate the alert. In the second part I will look at relative speeds, and combine the two to arrive at an alerting distance. I ignore vertical distance here, I leave that up to you.
According to the SafeSky website, SafeSky does not rely on sensors on-board, instead it collects aircraft surveillance data from various sources and transmits that data over a mobile phone connection to an App in the cockpit.

The latency of the whole alerting chain is unclear from SafeSky's website, but let's assume it takes 5 seconds1:

between the GPS on-board of an intruding aircraft determines it's position
forwards it to the transponder which then sends it out as an ADS-B message
to an ADS-B receiver which decodes it, and
forwards it over the internet to an ADS-B collection site
which processes it and forwards it to the SafeSky servers,
which decode it and determine which subscribers need to be informed,
then again forward it to over the internet and
the mobile network to your 2G mobile phone connection, and
then the SafeSky app on-board compares it to your GPS position
determines that there is a need to alert the pilot and then,
it both visually and aurally alerts the pilot of the intruder

Then, the pilot reacts to the alert by looking at the App's, and interprets the situation on the display. I assume this takes 3 seconds.

The pilot then looks outside to visually acquire the intruding traffic, understand the relative motions and determine the best avoidance manoeuvre. Let's say this takes 5 seconds.

The pilot initiates the avoidance manoeuvre and due to the dynamics of the aircraft, it takes 10 seconds to move out of the danger zone.

The above yields 23 seconds of latency, which I will conservatively stock up to 30 seconds for my own peace of mind2.

For the second part, let's take a look at the geometry.
A quick Google on geometry of mid-air collisions for GA gives me the following histogram:

source: Midair collisions: Morris, Carl. (2005). Midair collisions: Limitations of the see-and-avoid concept in civil aviation. Aviation, space, and environmental medicine. 76. 357-65.
I recommend reading the article as it gives a good analysis of typical midair collision scenarios and their contributing factors.
As you can see (and as expected), most in-pattern collisions are head-tail and acute, and will include many of the the infamous low-wing descents onto high-wing collision geometries. These accidents will have low relative speed.
Out-of-pattern, the acute geometry is prevaillent, but head-on and obtuse geometries combined account for the majority of mid-air geometries.
If you want to be conservative, you account for the perfect head-on scenario and calculate with the sum of both aircraft's speed.
Assuming the head-on intruder aircraft is a jet travelling towards you at 275 knots true airspeed (KTAS), and you yourself are flying at 150 KTAS, we end up at a relative speed of 425 knots.
In 30 seconds, you will have gotten 425 / 3600 * 30 = 3.54 NM closer to each other. And so a setting of 3.6 NM should be a good starting point for such a scenario.
However, if you are flying in an airspace with virtually no fast traffic and you yourselve are traveling at lower speeds, you can adjust the alerting distance to reduce the number of nuisance alerts. Let's say you are doing 120KTAS and so does everybody else, you could reduce to 240/3600 * 30 = 2 NM.
If this approach is still giving too many nuisance alerts, you can make educated adjustments. For example, you can argue that for head-on and obstuse scenarios, the intruder is in your primary field of vision, and may likely be caught with standard see and avoid techniques, and can be easily tracked during avoidance manoeuvring. Instead of taking 5+10 seconds for visually acquisition of the traffic and manoeuvring, you may be comfortable with a total of 10 seconds.
Here you are trading nuisance level with risk level. It's your judgement call: too many nuisance alert will cause you to eventually ignore even important alerts, but too low of a look-ahead time will not give you enough time to react.

IMPORTANT
As a final word of caution, and I can't stress this enough: please don't rely fully on a technology for traffic avoidance. Current traffic detection technologies are not reliable enough to ensure safe flight on their own. They are there to complement See-and-avoid. See-and-avoid is your primary way of noticing  and avoiding other traffic when flying VFR and even in IFR flight, S&A has its place!
Adding traffic detection technology can improve your See & Avoid performance, but be very aware of the following:

the SafeSky technology (as most other similar traffic detection systems) relies on other aircraft being equiped with some type of transmitting device to be visible. Not all aircraft are equiped with supported transmitters.
Even if other traffic is using a technology supported by SafeSky, it is not guaranateed that SafeSky will receive the data from that aircraft. Not all aircraft are within coverage of the receiver network.
Even if SafeSky receives the data from the other traffic, the connection to your device may be broken, or lack the performance to inform you in time.

As such, you have to keep telling yourself that you see only a partial traffic picture on your device. You have to keep looking outside.
This is easy if you often notice aircraft that are not shown on your traffic display, but as the technology improves, your mind will start to trust the technology, even though it is still unreliable.
If the technology tries to kill you once an hour, you will be on top of your game and probably fly safely for thousand hours. But if the technology tries to kill you only once every 100 hours, you will become complacent and need a lot of luck to make it to a thousand.

1The exact architecture and latency will be different for each surveillance source. Also, I have no knowledge about SafeSky sensor network nor processing logic. This is a very rough estimate.
2As always: since this a quick internet answer, the numbers above are not validated by thorough technical analysis and expert reviews as would normally be done when performing this kind of alerting threshold designs in my area of work. The numbers are merely made up by me, based on my experience, gut feeling and sometime failing memory. Don't just trust everything I write here, please do your own validation.

Thanks for the answer DeltaLima, this was a great read.... S&A is of course, important, but you cannot see behind you and either above or below you depending on wing set-up, so as the linked article mentions, it is a limiting technique :) Many thanks again!

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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Is medical certification required for flight attendents?
In the history of stewardesses: "In the earliest days of commercial flight, there were teenage cabin boys, and the first female stewardesses had to be registered nurses."
The 2009 book Flying Across America: The Airline Passenger Experience by Daniel L. Rust says, "*Dressed in hospital whites or military-style uniforms, a 'sky girl' of the 1930s not only served meals and soothed nerves...*
Is any type of medical certification required for flight attendants in the US or other parts of the world?

I don't think they were "required" to be nurses, just that they were nurses. There were very little if any "requirements" back then for cabin crew by the Civil Aeronautics Agency (CAA, precursor to the FAA) or the Bureau of Air Commerce which was in effect when the first flight attendants were used in the early 30's.

Good question, although for clarity you might consider rewording "medical certification" to "medical qualifications". When I read your question, I first thought you were asking if cabin crew need an FAA medical.

@ Ron Beyer - I have seen several documentaries that report the Airlines REQUIRED it as a condition of hire. In those days, a "nurse" could be someone who simply passed a Red Cross course (1 or 2 days) and there was no shortage of qualified "nurses" after WWI and WWII.

One of the first flight attendants anywhere, Ellen Church was also a registered nurse and certified pilot. She was unable to find work as a commercial pilot but she did have another influence on the nascent flight attendant industry: she convinced a major airline to hire only nurses.

She approached BAT (the forerunner of United Airlines) looking for a pilot's job, a futile hope for women in those days. But the BAT exec did like Church's other suggestion: that commercial airliners carry nurses on board.
Smelling a publicity coup, and figuring that on-board nurses would help quell the public's fear – very real at the time – of flying, he sold her proposal to the boys at the top. BAT hired eight nurses, including Church, for what it thought would be a three-month experiment.

These early flights attendants bore little resemblance to their modern counterparts, however

They were expected to haul luggage, screw down loose seats, help with fueling the plane and finally, at day's end, help the pilots push the plane into the hangar.

In modern aviation, trying to hire a registered nurse as a flight attendant doesn't seem to be a requirement. Here's Delta's page on hiring and it says

While specific positions will have unique requirements (example: Flight Attendant minimum age requirement is 21 years of age; see individual job descriptions), you must possess at least a H.S. diploma/GED equivalent, be at least 18 years of age, and possess current authorization to work in the U.S. to work at Delta.

The individual job descriptions are for language knowledge. None list a nursing degree as a requirement.
What changed? Well, mainly the medical profession. Nurses today have a LOT more educational requirements. Additionally, a lot more places use nurses. So nurses are in high demand now. Adding that requirement onto a flight attendant job would make the position more expensive and harder to fill.
Additionally, there might be legal liability. While simple medical devices (like AED devices to restart a heart) supplement and can prove of some limited use, imagine a flight attendant having a nursing degree and someone has a medical emergency or dies on their aircraft. Suing the airline would be a tempting. Legal liability is a very real problem here

A colleague of mine, however, was not as fortunate and encountered a much more serious condition -- urosepsis -- on a flight to London. [snip] My colleague missed her connecting flight, as did many passengers, and the other physician had to accompany the patient to the local ER because the emergency medical service worker did not want to be held liable for the IV line the ICU physician had inserted before the plane landed.

And

Unfortunately, liability is a very real concern, particularly in scenarios when you're not practicing in your regular clinical setting. Generally, physicians are covered by the Aviation Medical Assistance Act of 1998, which protects physicians who provide in-flight emergency medical assistance in the same way that state Good Samaritan laws do. It goes without saying that you do not have to volunteer if you do not feel comfortable with the situation or are concerned for your own safety. If you have had a few drinks or have taken sedating medications, you need to use your judgment about whether to respond.
Since you are volunteering and not technically on duty, under the 1998 law, you are covered as long as you don't engage in any willful misconduct or commit gross negligence. You should make yourself aware of the law's limitations. Many physicians feel a moral obligation to help in a medical emergency, and, thus, we can find ourselves in these situations.

If passengers leap into action to help a distressed patent, the airline avoids liability.

Two comments on the urosepsis scenario: 1) The paramedic who insisted on the doctor accompanying the patient to the ER because of the IV was a first-class weenie. 2) I wonder where the heck he got IV supplies on board an airplane? That's not something doctors or airlines carry.

@CareyGregory I believe it's possible to rig one up just using salt, water, bottles and straws (it's not advisable in normal circumstances, but at 30k feet advisable gives way to necessary)

That might be possible on a TV show, but it's just not possible in real life. At a minimum the doctor had access to an IV catheter, tubing, and IV fluids. Maybe somebody on board happened to be carrying such things as an extraordinary coincidence.

Flights often are required to carry an emergency medical kit, with the intention that medically qualified passengers could use it in consultation with doctors on the ground by satellite phone (airlines subscribe to a service, e.g. MedLink). In the US, the contents of that kit must include IV supplies and saline solution.


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Could future airplanes use rockets to recover plane and/or prevent crash landings?
With the advanced rocket technology that SpaceX uses in the Falcon for re-entry and controlled descent, could the same technology be used in commercial aircraft? I'm asking about using it in emergencies like engine failures, stalls, loss of pitch control etc. in order to prevent a crash, or at least minimize the damage and reduce the fatality rate.
For fuel storage, the rockets could burn what's available in the tank without any need for an auxiliary tank dedicated to them.
I know that there is a similar technology used in the military: the C-130 Rocket Assisted Takeoff (JATO). But it's only used for short runway landings and take-offs.
EDIT:
The question is specific to thrusters that could recover the airplane from  unrecoverable situations (dive, bank, upside down,...). Question Why are movable rocket thrusters not used in airplanes? answer is generic and not detailed for such use case.

I don't see why that would even need to be used since an airplane without engines is a glider and gliders fly well. And about the "stall" you are talking about, are you talking about compressor stall or wing stall?

I talk about wing stall. I updated the question to include a catastrophic scenarios like the loss of pitch control accured in the Alaska Airlines Flight 261 : https://en.wikipedia.org/wiki/Alaska_Airlines_Flight_261

What exactly is the Falcon controlled descent technology that you mentioned? Are you asking if airliners could use rockets for emergency landings?

where would you fit the extra equipment required? who would pay for the extra cost caused by the extra weight?

@Federico, well that's the question, is it technically and financially feasible     in the futur?

@Pondlife yeah, and also recover the plane in case of unrecoverable dive for exemple.

This might be a partial dupe of this question, since using rockets implies that an oxidizer would be needed.

http://aviation.stackexchange.com/questions/11584/why-movable-thrusters-are-not-used-in-airplanes/11589#11589

@inaliahgle the most upvoted answer on that linked question gives the answer, even though the question is slightly different.

Rockets are machines, and so are subject to failures.  They also tend to carry a lot of rather explosive fuel: see for instance the recent SpaceX explosion, or the explosion of the Pepcon rocket fuel plant back in 1988.  So it seems that putting emergency rockets on planes might easily cause more accidents than they prevent.

They could but there really is no reason to do so. Most aircraft glide just fine if they lose all engine thrust and are perfectly controllable in such situations (provided they can actuate the control surfaces). As for the situation you propose there are a few issues with it,
First off, rocket fuel is not jet fuel and can't just be burned in the same way.
The JATO engines are a bit different than the rocket assist that Space-X is using (although the principal is similar). For takeoff the Jato engines help add to the totally available thrust and allow the plane to get off the ground in a shorter distance. The rockets are (I believe) single use and provide a very quick burst of thrust.
As for the loss of control this depends on what causes the loss of control and how the rockets are affixed to the airframe. In the case of the Alaska airlines flight you reference the jack screw forced the tail into a position and prevented it from moving, this put the plane in an unrecoverable dive that would only be exacerbated by rocket engines unless you could control the angle at which they were pointed. Again in the outlined case you would need to be able to counteract the full surface deflection of a control surface. In essence you would need the kind of thrust control that space ships have used over the years.
Now to answer the broad question of "could rocket thrusters be used to bring a disabled aircraft down softly" the short answer is yes. They could be added for such a task but the toll they take on the plane would make it a hard sell to the airlines. In short the use case is small and the added weight cuts into the profitability of the flight (you need a lot of jet fuel and heavy thrusters to pull this off). On top of all that the system would have to be built and certified for use, a task that should not be taken lightly. In the end you get a safety system that has a small use case and a big impact on the aircraft. Perhaps the best thing airplanes can learn from space craft landings is the use of parachutes.
The last and somewhat debatable point (common to all safety systems) is when do you actually turn it on? What is your definition of unrecoverable (yes this varies)? Cirrus teaches people to basically always pull the chute in an emergency. Are you going to teach pilots to to always trigger this rocket decent system even if the plane may be recoverable? What if you run out of fuel before getting to the ground and now you are to low to recover from the rocket assisted decent?

Well, the idea isn't new.  During the Iran Hostage Crisis of 1979, an secret rescue attempt codenamed Operation Credible Sport, which would have utilized a one-of-a-kind YMC-130 Hercules aircraft fitted with downward and forward firing JATO rockets, which were planned to allow the aircraft to land in a soccer stadium near where the hostages were being held.  It crashed during a test flight on Oct 29, 1980 due both to equipment malfunction and flight crew mishandling.  The project was scrapped shortly thereafter due to successful negotiations with Iran on the part of the Reagan Administration obtaining the release of the hostages.

As to whether this kind of a technology would ever be added onto a commercial aircraft, well the YMC-130 was partly a failure and partly a success.  It was a failure in the sense that there was never a fully successful test flight resulting in a crash, but it was a success in the sense that the crew walked away from the accident, perhaps a prophetic picture of what would happed to a commercial airline outfitted as such.
I doubt the airlines would be interested in the system for the following reasons:

The system is going to add a lot of excess weight and drag onto an existing airframe or even one specifically designed for this - weight which is not paying its way on each flight and burning extra fuel to lug it around on each flight.
These systems also require maintenance and overhaul to work properly, adding additional costs.
This in an embryonic technology without a lot of flight time behind it and one disastrous flight test; not exactly a great selling point to airline executives.
As airline accidents already have over a 90% survivability at current time, such a system seems unwarranted for deployment into commercial aviation.


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How is the glide ratio affected when converting a wheeled plane to floats?
I'm looking for published data that describe the effect on glide ratio when an aircraft is converted from wheeled to float configuration.  I suspect that the glide ratio goes down considerably due to all the drag from the floats and attachments, but an internet search turned up nothing.
I'm particularly interested in DeHaviland Beavers and Otters (which commonly have their gear switched between wheels and floats), but any source would be appreciated.

As glide ratio is the same as the L/D ratio, the increase in drag due to the addition of floats will reduce it. The Beaver's POH gives the effect of addition of floats in the glide ratio:

DHC-2 Beaver's Glide ratio; image from DHC-2- Beaver POH
Note that at their configuration for best glide ratio, there is a drop in about 10% in glide ratio for the configuration for floats. This will differ for different configurations and operating conditions, but the basic effect of addition of floats will be a reduction in glide ratio.


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Can civilians who buy ex-military fighter jets get the necessary maintenance information for them?
I was looking for fighter jets which are available to be bought by private citizens. I was wondering: how would you schedule the maintenance, as most of the jets are ex-military and there are no maintenance plans?

My suggestion would be to ask the manufacturer to provide you with the maintenance manuals.

and, maybe find a retired military maintenance officer or senior NCO and hire him.

Some fighters with larger followings, like the L-39 Albatross, you can find service centers in different parts of the country to service them.

Actually for airworthiness certification of ex-military jets I believe they require you to obtain any/all declassified manuals, maintenance manuals, etc. on them. I know for a FACT to certify the Mig-21 they require you to own all the Mig-21 manuals (technical orders, maintenance manuals, parts catalogs, engine manuals, inspection schedules, flight manuals, and checklists) sigh I should have bought that mig. Biggest regret of my life.

And from these manuals it should be possible to have parts custom-fabricated; that's actually not difficult to get done it's just expensive.

Most military surplus aircraft are considered by the FAA to be experimental exhibition aircraft and subject to the rules and maintenance requirements for experimental aircraft.  That being said, most owners seek out retired military personnel who did maintenance on these aircraft and currently work as AIs (authorized inspectors) in civilian aviation to handle maintenance and repairs.  Firms like Code 1 Aviation and Thornton Aircraft specialize in certain types of military aircraft and have collected libraries of maintenance manuals and other resources for owners and can perform maintenance conforming to the old military and civilian experimental aircraft guidelines.

And what about the maintenance certificates for the mechanic? I read a lot of military aircraft mechanics do not get the kind of license a civilian mechanic has.

No, I’m talking about ex-military mechanics which obtain an AI from the FAA.

What exactly is an AI?

Authorized Inspector